In automobile engines with pressure wave superchargers, the peak pressures in the cylinders should be in the vicinity of the allowable maximum combustion pressure over a wide rotational speed range. In order to attain this desired condition, a pressure wave supercharger for such engines is so designed that it generates a supercharge pressure under load at the maximum engine rotational speed which, without blow-down of the exhaust gas, is higher than the supercharge pressure necessary to generate the allowable peak pressure. By means of exhaust gas blow-down, which is controlled for example by the supercharge pressure, a higher allowable supercharge pressure and, therefore, more favorable variations of torque and fuel consumption can be attained over a wide operating range. A vehicle equipped with such an engine can therefore be driven without much gear changing.
Blow-down of exhaust gas does not have to be employed in the case of utility vehicle engines because they permit higher supercharge pressures and operate over a narrower rotational speed range than passenger car engines. In the case of utility vehicles, therefore, it is sufficient so to design the pressure wave supercharger that it supplies the maximum supercharge pressure for which the peak value of the combustion pressure is still allowable over the desired operating range.
A control device for controlling supercharge pressure by means of appropriate blow-down of the engine exhaust gas before the pressure wave supercharger is known from the European patent application No. EP-Al 0 123, 990. The valve element used in the device there described, by which the blow-down duct (known to experts as the wastegate) is more of less freed or closed, is a spring loaded plate valve. In a further device, described in the Swiss Pat. No. 398,185, for controlling the supercharge pressure by blow-down of exhaust gas and in which the exhaust gas pressure is used for actuating the valve element, the valve element is again spring loaded and is mushroom-shaped in the region of the sealing elements.
The springs of such plate and mushroom valves are heavily thermally loaded by the exhaust gases passing by the valve, which loading alters their spring constants in the course of time so that the valve opens earlier. The blow-down pressure in the supercharger and hence also the combustion pressures are reduced. The result, therefore, is loss of power in the operating range of the engine mentioned at the beginning.
In order to avoid these disadvantages of the two blow-down devices previously mentioned, use has been made--instead of the spring loaded plate and mushroom valve mentioned--of a flap valve which does not require a spring in the blow-down range and is actuated by exhaust gas pressure or, as described in the Swiss patent application No. 2355/83-7 mentioned, by another suitable process pressure. The actuation force then acts on a lever rotationally stiffly connected to the flap valve. Such a flap valve is substantially cheaper to produce than the two known types of valves mentioned above. In addition, no change to the opening characteristic occurs with such a flap valve because the spring is no longer required. However, with the aforementioned plate and mushroom valves the excess exhaust gases flow symmetrically about the valve's longitudinal axis and substantially parallel to the flow of the low pressure exhaust gases, i.e. of the exhaust pipe gases in the exhaust gas outlet stub pipe. In contrast, the blow-down exhaust gases in a flap valve unfortunately flow into the exhaust gas outlet stub pipe with a velocity component transverse to the flow direction of the outlet pipe gases, which produces the disadvantages described below.
For satisfactory and effective functioning of the pressure wave supercharger, the expanded exhaust gases, after they have carried out their compression work, must be completely scavenged into the exhaust gas outlet stub pipe together with the mixture of air and exhaust gas, which has formed in the mixing zone, i.e. in the region of the separating surface between the air and exhaust gas. This scavenging is supported by the induction air, which enters the rotor cells on the side opposite to the exhaust openings, the rotor being simultaneously cooled by it. In order to obtain satisfactory compression efficiencies, however, even more thorough cooling of the rotor is necessary. For this purpose, the pressure wave supercharger must induce more air than the quantity of compressed supercharge air supplied to the engine. This additionally induced air is called scavenge air and the ratio of the scavenge air flow to the supercharge air flow is known as the "scavenging coefficient" of the pressure wave supercharger. This scavenging coefficient decreases with increasing engine rotational speed and decreasing engine load.
In a pressure wave supercharger, blow-down through the wastegate results mainly, as in a turbo-charger, in a deterioration of the overall efficiency and hence the specific fuel consumption but not in the scavenging coeffficient because the scavenging energy decreases approximately in proportion to the compression energy.
In the case of small blow-down flows, the transverse component of the flow into the exhaust duct has no important adverse effect on the exhaust gas flow and, therefore, on the scavenging coefficient. In the case of larger blow-down flows, however, the scavenging deteriorates noticeably because of the large transverse component of the entry velocity and hence the compression efficiency is also adversely effected.